Sulfur and sulfides in chondrules

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ScienceDirect Geochimica et Cosmochimica Acta 119 (2013) 117–136 www.elsevier.com/locate/gca

Sulfur and sulfides in chondrules Yves Marrocchi ⇑, Guy Libourel Universite´ de Lorraine, CRPG, UPR 2300, Vandoeuvre les Nancy F-54501, France CNRS, CRPG, UPR 2300, Vandoeuvre les Nancy F-54501, France Received 11 July 2012; accepted in revised form 15 May 2013; available online 24 May 2013

Abstract The nature and distribution of sulfides within type I PO, POP and PP chondrules of the carbonaceous chondrite Vigarano (CV3) have been studied by secondary electron microscopy and electron microprobe. They occur predominantly as spheroidal blebs composed entirely of low-Ni iron sulfide (troilite, FeS) or troilite + magnetite but in less abundance in association with metallic Fe–Ni beads in opaque assemblages. Troilites are mainly located within the low-Ca pyroxene outer zone and their amounts increase with the abundance of low-Ca pyroxene within chondrules, suggesting co-crystallization of troilite and low-Ca pyroxene during high-temperature events. We show that sulfur concentration and sulfide occurrence in chondrules obey high temperature sulfur solubility and saturation laws. Depending on the fS2 and fO2 of the surrounding gas and on the melt composition, mainly the FeO content, sulfur dissolved in chondrule melts may eventually reach a concentration limit, the sulfur content at sulfide saturation (SCSS), at which an immiscible iron sulfide liquid separates from the silicate melt. The occurrence of both a silicate melt and an immiscible iron sulfide liquid is further supported by the non-wetting behavior of sulfides on silicate phases in chondrules due to the high interfacial tension between their precursor iron-sulfide liquid droplets and the surrounding silicate melt during the high temperature chondrule-forming event. The evolution of chondrule melts from PO to PP towards more silicic compositions, very likely due to high PSiO(g) of the surrounding nebular gas, induces saturation of FeS at much lower S content in PP than in PO chondrules, leading to the cocrystallization of iron sulfides and low-Ca pyroxenes. Conditions of co-saturation of low-Ca pyroxene and FeS are only achieved in non canonical environments characterized by high partial pressures of sulfur and SiO and redox conditions more oxidizing than IW-3. Fe and S mass balance calculations also suggest the occurrence of an external source of iron, very likely gaseous, during chondrule formation. We therefore propose that enrichments in sulfur (and other volatile and moderately volatile elements) from PO to PP type I bulk chondrule compositions towards chondritic values result from progressive reaction between partially depleted olivine-bearing precursors and a volatile-rich gas phase. Ó 2013 Elsevier Ltd. All rights reserved.

1. INTRODUCTION Sulfur is the tenth most abundant element in the solar system with an atomic abundance close to that of Fe and estimated at N(S)  4.5  105 (normalized to the number of silicon atoms ofN(Si) = 106) (Lodders, 2003). The average chondrite contains about 2 wt% S (Anders and Grevesse, 1989), occurring as stoichiometric troilite (FeS) or pentlandite ((Fe,Ni)9S8) blebs (Grossman and Wasson, ⇑ Corresponding author.

E-mail address: [email protected] (Y. Marrocchi). 0016-7037/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.gca.2013.05.020

1985; Rubin et al., 1999). In unequilibrated ordinary chondrites (hereafter UOCs), sulfides appear to be ubiquitous as they occur either disseminated in the matrix or located within or near the edge of chondrules (Grossman and Wasson, 1985; Zanda et al., 1995; Rubin et al., 1999). Within UOC chondrules, troilites are always reported in association with Fe–Ni metal and oxidized minerals in the so-called opaque assemblages (hereafter OAs) (Blum et al., 1989; Zanda et al., 1995; Rubin et al., 1999; Tachibana and Huss, 2005). The origin of these troilite grains is still a matter of debate due to the volatile behavior of sulfur under canonical conditions (Lauretta et al., 1996).

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Rubin et al. (1999) defined primary troilites (i.e., that were present among chondrule precursors) as those associated with igneous OAs located within the inner half-radius of the chondrules, and embedded in silicate phenocrysts. Based on these petrographic arguments, they concluded that  13% of chondrules (from a database of eight ordinary chondrites) might contain primary troilites. (Tachibana and Huss, 2005) extended the criteria for primary troilite to all FeS associated with metal spherules located within chondrules (i.e., that are not in direct contact with the chondrule surface). In this scenario, troilites within chondrules are primary minerals that partially escaped evaporation during the high temperature chondrule-forming event (Rubin et al., 1999; Tachibana and Huss, 2005). However, experimental studies suggest that FeS should have been vaporized under thermal conditions expected for chondrule formation, even for extremely fast cooling rates (Hewins and Connolly, 1996; Hewins et al., 1997, 2005). Based on the occurrence of compositional zoning (e.g., Na, K, Fe and Si) in the chondrule mesostases, it has been proposed that SiO2, MgO and alkali elements could have recondensed during cooling into the silicate melts from which they evaporated (Alexander et al., 2000; Ozawa and Nagahara, 2001). However, the behavior of sulfur is thought to be different as it would not interact with the chondrule melt due to its low condensation temperature under canonical conditions (i.e., FeS through a reaction between H2S and Fe–Ni metal at 713 K) (Lauretta et al., 1996). Instead, it has been proposed that sulfur would have recondensed as sulfidic veneers on the surface of the solidified chondrules (Zanda et al., 1995). However, the sulfur isotopic compositions of putative primary troilites preserved within chondrules and sulfidic veneers do not show any measurable differences contrary to what would be expected for such a process (Tachibana and Huss, 2005). Thus, it is generally proposed that sulfur underwent important secondary redistribution during solar nebula and/or parent-body processes such as redistribution of sulfur during thermal metamorphism (Blum et al., 1989; Zanda et al., 1995), hydrothermal alteration (Krot et al., 2004), shock metamorphism (Rubin, 1992), or reactions between Fe–Ni metal and a sulfur-rich gas (H2S) (Lauretta et al., 1996, 1997a; Schrader and Lauretta, 2010). The origin of sulfides thus remains poorly constrained. If the FeS are primary phases, they would provide important clues on the high temperature thermal regime of chondrules (Tachibana and Huss, 2005; Hezel et al., 2010). Alternatively, a secondary origin of troilites would reflect variable physical and chemical conditions in the accretion disk or on the parent-body (Zanda et al., 1995; Lauretta et al., 1996; Rubin et al., 1999). Interestingly, it is possible that sulfides have recorded both processes and thus they could be used as a powerful probe for understanding the formation of chondrules and their secondary evolution (Rubin et al., 1999). Hence, understanding sulfur behavior during chondrule formation could bring important constraints on the nature and the chronology of the processes that affected chondrules. In this work, we report a systematic petrographic and mineralogical survey of sulfides in type I chondrules of

the carbonaceous chondrite Vigarano (CV3) in order to better establish the conditions under which they formed. Vigarano was chosen because it is part of the reduced CV clan that is thought to have undergone only a very low degree of alteration compared to the oxidized sub-group (Krot et al., 1995). In addition, thermoluminescence sensitivity data (Symes et al., 1993), the noble gas characteristics of nanodiamonds (Huss and Lewis, 1995) and Raman spectroscopy (Bonal et al., 2006) indicate that Vigarano has been only mildly metamorphosed (i.e., 3.1–3.4). Furthermore, Vigarano is a sulfur-rich chondrite although it has been assumed, based on the budget of presolar grains and the most volatile elements in the matrix, that the precursors of CV chondrules did not contain any sulfides (Zanda et al., 2009). These characteristics make Vigarano a good candidate for deciphering the behavior of sulfur during the formation and secondary evolution of chondrules of primitive meteorites. 2. ANALYTICAL PROCEDURE We surveyed all type I chondrules in two thin sections of Vigarano (Vigarano 477-2 and Vigarano-P). Chondrules were examined microscopically in transmitted light and reflected light. Scanning electron microscope (SEM) observations and Energy Dispersive X-ray (EDX) spectral analyses were performed at CRPG using a JEOL JSM-6510 equipped with an EDX Genesis X-ray detector, using a 3 nA primary beam at 15 kV (Figs. 1 and 2). We also performed multi-element EDX mapping (Mg, Si, Fe, Ni, S, Na, Ca, and Al) of 25 selected chondrules of all petrologic types (Figs. 1 and 2). The SEM was calibrated using silicate, metal and sulfide standards before the acquisition of each map over 6 h. After acquisition, the Si, Mg, Fe, Ni and S maps were combined into a single image in order to highlight the mineralogical characteristics of the chondrules: olivines, low-Ca pyroxenes, Fe–Ni metal, iron sulfides and mesostasis were identified. The modal abundance of each chondrule was determined using the imageJÓ software following the modal recombination technique. The chondrules were cut out of each map manually by excluding the matrix and the rim on the basis of the iron and magnesium content of silicates and the area (number of pixels) of each chondrule was determined. The area of each mineralogical phase was then determined by adjusting the color distribution in order to distinguish the phases. The modal abundance corresponds to the ratio of the area (number of pixels) of a given mineralogical phase to the total chondrule area. Using the same protocol, modal abundances within individual chondrule were determined for the inner part, mainly composed of olivines, and the outer zone composed of low-Ca pyroxenes. This procedure was repeated for four POP chondrules that present well-defined zonations. In order to estimate the uncertainties involved, modal abundances were determined twice on each chondrule, using two different operators (Hezel, 2007; Hezel and Kießwetter, 2010). It is important to note that, following this approach, we considered only sulfides located within the chondrules and excluded those present in the matrix as well as those located in the fine-grained rims of the chondrules.

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Fig. 1. (a and d) Back-scattered electron images of two POP chondrules (Ch-05 and Ch-06; Vigarano-P), in which sulfide distribution and chemical composition of the mesostasis were determined. (b and d) EXD elemental maps of sulfur (purple) distribution in the two POP chondrules. (c and e) Compiled EDX maps of Si and Mg of the two POP chondrules revealing the mineralogy of the different chondrule silicate phases: olivines (orange), low-Ca pyroxenes (pale green) and mesostasis (dark green). These maps show that sulfides are mainly associated with low-Ca pyroxenes mainly in the outer zone of chondrules.

Prior to electron microprobe analysis, high magnification observations of the chondrule mesostases were performed with a FE-SEM JEOL-JSM-7600F operating at 15 keV with a 10 nA primary beam. Quantitative analyses of the mineralogical compositions of chondrules were made with a CAMECA SX-50 electron microprobe (CAMECA, Paris, France) at the University of Paris VI. A 10 nA focused beam ( 2 lm), accelerated to 15 kV potential difference, was used for spot analyses of silicates, oxides, metals and sulfides with 20 s analysis times. For the analysis of sulfides, we used Fe, Ni, Co, and Cr-metals, and pyrite and scheibersite as standards for Fe, Ni, Co, Cr, S and P, respectively. The detection limits obtained were estimated at 0.03 at% for Fe, Co and P, 0.08 at% for Ni and 0.006 at% for S and Cr. The mesostasis was analyzed using a 20 nA defocused beam ( 15 lm) with a 15 kV accelerating voltage with analysis times of up to 60 s

for the trace elements. To reduce the loss of volatile elements (i.e., Na, K and S), these were analyzed first. The PAP software was used for matrix corrections (Pouchou and Pichoir, 1984). Detection limits in silicates were 0.03 wt% for SiO2, Al2O3, CaO and MgO; 0.04 wt% for Na2O, K2O, TiO2 and P2O5; and 0.07 wt% for NiO, Cr2O3 and FeO. Special care was taken for the determination of the mesostasis sulfur content by: (i) working with a high beam current of 20 nA, (ii) increasing the analysis time to 100 s and (iii) measuring the background on each side of the sulfur peak for 50 s per side (O’Neill and Mavrogenes, 2002). This protocol allows the sulfur detection limit to be lowered to  75 ppm. It is important to note that we report the sulfur concentration as wt% (or ppm) S instead of wt% SO3 (Wallace and Carmichael, 1992; O’Neill and Mavrogenes, 2002).

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Fig. 2. (a and d) Back-scattered electron images of two PP (Ch-44 and Ch-46; Vigarano-P) chondrules. Ch-46 is similar to non-spherical lobate chondrules described by (Rubin and Wasson, 2005) (b and d) EXD elemental maps of sulfur (purple) distribution in the two PP chondrules. (c and e) Compiled EDX maps of Si and Mg of the two PP chondrules, showing the mineralogy of the different chondrule silicate phases: olivines (orange), low-Ca pyroxene (pale green) and mesostasis (dark green). Sulfides are present across all these PP chondrules in higher proportions than in the PO chondrules.

3. RESULTS 3.1. General characteristics of the chondrules We examined all the type I chondrules, including fragments, in two sections of Vigarano. All the textural types are represented (i.e., PO, POP, PP and non-spherical lobate chondrules; Figs. 1 and 2). Type I porphyritic chondrules are characterized by small grains of low-FeO olivine ( 30–100 lm), slightly larger low-Ca pyroxenes ( 60– 150 lm), glassy mesostasis and Fe–Ni metal beads. Olivines present rounded to subhedral shapes frequently associated with a glassy mesostasis that might contain small Ca-pyroxene crystallites and/or evidence of devitrification. In contrast, low-Ca pyroxenes are in the form of euhedral crystals, with resorbed or poikilitically enclosed olivines and with little mesostasis. Most of these chondrules are radially zoned with olivines and mesostasis

located towards the interior of the chondrules while the outer zone is dominated by low-Ca pyroxenes parallel to the surface (Scott and Taylor, 1983; Tissandier et al., 2002). 3.2. Sulfides in type I chondrules Sulfides in type I porphyritic chondrules of Vigarano are present only as stoichiometric troilite blebs (FeS). No pyrrhotite or pentlandite was observed in the two sections we surveyed. Most of the sulfide blebs are composed entirely of troilite or are associated with magnetites (Fig 3). Troilites associated with Fe–Ni metal are also observed in Vigarano (Fig. 4) but in low abundance compared to ordinary chondrites, in which all troilites are part of OAs (Rubin et al., 1999). These troilites are located inside or at the edge of solid metal spherules and they present low but variable nickel contents (0.1–0.65 at%; Fig. 4 and Table 1) (Rubin

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Fig. 3. Back-scattered electron images and corresponding sketches of different type of sulfide commonly observed in Vigarano. (a) Massive blebs of sulfide in complex intergrowth with low-Ca pyroxenes and small pockets of mesostasis at the rim of the chondrule (Ch-00 in Vigarano-P). Small rounded olivines are also present as inclusions in low-Ca pyroxenes. (b) Spheroı¨dal droplets of troilite (± magnetite) poikitically enclosed in low-Ca pyroxenes (Ch-00 in Vigarano-P). (c) Ameboı¨dal sulfide pools located in mesostasis pockets and adhering to rounded olivine (Ch-04 in Vigarano-477-2). (d) Troilite + magnetite ameboı¨dal structure located in a pocket of mesostasis in contact with lowCa pyroxene (Ch-18 in Vigarano-477-2; see text for discussion).

et al., 1999; Tachibana and Huss, 2005). In contrast, the nickel contents of blebs composed either entirely of troilites or of troilite associated with magnetite are below the

detection limit (Fig. 4 and Table 1). These parageneses do not present the numerous holes that are commonly observed in OAs. Hereafter, the term troilites will refer only

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Fig. 4. (a) Back-scattered electron images of a PO chondrule (Ch-04; Vigarano-477-2). The red box corresponds to the ameboidal sulfide pool shown in Fig 3c. (b) Compiled EDX maps of Si, Mg and S of the chondrules showing the mineralogy of the different chondrule phases: olivines (orange), low-Ca pyroxene (pale green), mesostasis (dark green) and the sulfur distribution (purple). (c) EXD elemental maps of sulfur (purple) distribution. (d) EXD elemental maps of nickel (yellow) distribution. These maps show that sulfides are not associated with Fe–Ni metal.

to these sulfides that are not associated with metallic Fe–Ni beads in opaque assemblages. Troilites, sometimes associated with magnetites, are present in all the chondrules but their distribution within the chondrules is variable. We observed FeS: (i) in association with low-Ca pyroxenes either as massive blebs (10– 200 lm, Fig. 3a) or as poikilitically enclosed droplets (10– 20 lm, Fig. 3b) and (ii) in mesostasis pockets as ameboidal sulfide pools (10–100 lm) adhering to rounded olivine and low-Ca pyroxene grains and/or to olivine–silicate melt junctions with large obtuse wetting angles (Figs. 3a, b and 4). No narrow sulfide trails along the silicate grain boundaries were observed. Our detailed survey of porphyritic chondrules using EDX mapping revealed that troilites are mainly located in the low-Ca pyroxene outer zone and that the amount of troilite increases with the abundance of low-Ca pyroxene (Figs. 1 and 2). This trend is confirmed by the observed correlation between the percentage of sulfides and low-Ca pyroxenes determined from image processing of the EDX maps of 25 chondrules of all petrological types (Fig. 5 and Table 2). Among chondrules, PO chondrules show the lowest sulfide concentrations, with sulfides generally located at the periphery of the chondrules (Figs. 1 and 5), while PP chondrules present the highest concentration of sulfides; these are homogeneously distributed throughout the chondrules (Figs. 2 and 5). Modal abundances determined from image processing of four chondrules divided in two zones, the inner zone mainly composed of olivines (PO-zone, Table 2) and the outer portion mostly composed of low-Ca pyroxenes (PP-zone, Table 2) show the same

trend with sulfides more abundant in the PP relative to PO parts of individual Vigarano type I chondrules. Sulfide modal abundance (between chondrules or within a given chondrule) is thus clearly related to the chondrule petrographic type (Figs. 1, 2 and 5), with an increase from PO to POP to PP. Though our survey reveals that troilites are very often poikilitically enclosed in low-Ca pyroxene, we observed no troilite inclusions in Mg-rich olivines (Fig. 3b). Finally, it is interesting to note that all of the broken chondrules we observed in the two sections present the same distribution with troilite blebs being located on the convex outer part of the chondrules in association with low-Ca-pyroxenes (Fig. 6). 3.3. Mesostasis compositions The compositions of the mesostasis of five wellpreserved type I chondrules (PO and POP) were analyzed by electron microprobe. The major element concentrations and compositional trends we obtained are consistent with those generally reported for chondrule mesostasis (Libourel et al., 2006). They present important interchondrule variations in chemical compositions (in wt%): 41 < SiO2 < 50, 12 < CaO < 19, 22 < Al2O3 < 33, 1.35 < MgO < 9.65, 0.2 < Na2O < 6.7, 0.21 < TiO2 < 1.17, 0.07 < S < 0.17 and 0.3 < FeO < 3.5 (Table 3). We also observed significant chemical variations of the major and trace elements inside the mesostasis of each chondrule (Table 3). Our results reveal the existence of simple compositional arrays in chondrules (Table 3): SiO2 is negatively correlated

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Table 1 Representative chemical composition of sulfides (at%) occurring as pure troilite (FeS), associated only with magnetite (FeS + Mt) or in association with Fe–Ni metal and magnetite in the so-called opaque assemblages (OA). bdl = below detection limit. Meteorite

Chondrule

Mineralogy

Fe

Ni

Co

S

P

Cr

Vigarano-477-2

Ch-04

OA OA OA FeS FeS + Mt FeS FeS FeS + Mt FeS

48.84 50.04 50.15 49.91 49.77 50.11 50.07 50.02 49.82

0.654 0.203 0.117 bdl bdl bdl bdl 0.161 bdl

0.063 0.059 bdl bdl 0.069 0.087 bdl bdl bdl

50.38 49.67 49.58 50.09 50.16 49.77 49.93 49.92 50.18

0.037 bdl 0.094 bdl bdl bdl bdl bdl bdl

0.026 0.041 0.056 bdl bdl 0.029 bdl bdl bdl

Vigarano-P

Ch-01

OA OA OA OA OA OA FeS FeS + Mt FeS FeS

49.21 49.91 50.07 49.69 49.72 49.37 49.87 49.71 50.02 49.84

0.292 0.158 0.371 0.216 0.131 0.301 bdl bdl bdl bdl

bdl 0.146 0.094 0.074 0.093 0.076 0.163 0.096 bdl bdl

50.41 49.66 49.35 49.85 49.92 50.14 49.97 50.17 49.92 50.12

bdl 0.046 0.071 0.101 0.052 bdl bdl bdl bdl bdl

0.084 0.084 0.047 0.064 0.086 0.068 bdl 0.025 0.064 0.037

Vigarano-P

Ch-03

OA OA OA OA FeS FeS + Mt FeS FeS

49.74 49.81 49.73 49.91 49.74 49.92 49.89 49.97

0.193 0.234 0.199 0.096 bdl bdl bdl bdl

0.087 0.061 0.074 0.081 bdl bdl bdl bdl

49.83 49.75 49.85 49.82 50.26 50.04 50.11 50.03

0.081 0.069 0.064 bdl bdl bdl bdl bdl

0.067 0.078 0.081 0.091 bdl 0.041 bdl bdl

to Al2O3 and CaO is negatively correlated to Na2O. Sulfur is present in all mesostases, in concentrations up to around 1700 ppm (Table 3) and it presents a positive correlation with FeO (Fig. 6). It is important to note that the correlation between FeO and S is present in all the chondrule mesostases we analyzed but with variable slopes (Fig. 7). 3.4. Iron sulfide wetting angles in chondrules The morphology and wetting angles of sulfide globules in contact with olivines/low-Ca pyroxenes and glassy silicate mesostasis have been studied in Vigarano chondrules. Troilites mainly occur as two types of metal pools with the coexisting crystalline olivines/low-Ca pyroxenes and the mesostasis: (i) spheroidal droplets (10–200 lm) in lowCa pyroxenes (Fig. 3a, b) and (ii) ameboidal sulfide pools (10–100 lm) located in mesostasis pockets and adhering to rounded olivine and low-Ca pyroxene grains and/or to olivine–silicate melt junctions with large obtuse wetting angles on the order of 160–170° (Fig. 3c, d). 4. DISCUSSION 4.1. Evidence for a high-temperature origin for chondrule troilites The abundance of low-Ca pyroxene relative to olivine increases from PO to POP and PP chondrules. Based on

the fact that these zonations involve high-temperature assemblages, it is generally accepted that they were acquired during the chondrule formation events, very likely by SiO nebular gas–melt interactions (Tissandier et al., 2002; Hezel et al., 2003). That troilites are systematically correlated with the modal abundances of low-Ca pyroxenes in type I chondrules of Vigarano (Fig. 5) and are poikilitically enclosed in low-Ca pyroxenes (Fig. 3b) suggests their cocrystallization with the low-Ca pyroxenes during hightemperature events. Several arguments support the fact that troilite blebs were inherited from the high-temperature forming event of type I chondrules and were not due to secondary alteration processes. Based on the observations of sulfides in coarse-grained low-FeO chondrules of Semarkona (LL 3.0) and more metamorphosed type-3 ordinary chondrites, (Zanda et al., 1995) and (Hewins et al., 1997) proposed that FeS in chondrules were extensively redistributed during thermal metamorphism on the parent bodies. Secondary sulfides obtained after heating a mixture of silicates, metallic Fe and sulfides at 500 °C and 900 °C are characterized by the formation of sulfidic rims around metallic Fe grains and narrow sulfide trails that connect these sulfide–metal associations along the silicate grain boundaries (Lauretta et al., 1997b). We did not find any evidence of such structures in either section of Vigarano examined, consistent with earlier reports of type 3.00 to 3.6 ordinary chondrites (Rubin et al., 1999). Sulfides are restricted to the convex

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1

PP POP

2

r =0.72

En/(En+Ol)

0.8

concentrations and abundance patterns of rare earth elements reported in troilites located within Bishunpur chondrules, which are similar to those reported for sulfides formed by high temperature igneous processes during laboratory experiments (Kong et al., 2000).

0.6

4.2. Sulfur solubility and iron sulfide saturation in chondrule melts

0.4

The amount of sulfur that can be incorporated into a silicate melt is a function of sulfur fugacity, oxygen fugacity, temperature, pressure and liquid composition (Mavrogenes and O’Neill, 1999; Holzheid and Grove, 2002; O’Neill and Mavrogenes, 2002; Backnaes and Deubener, 2011). Several experimental and theoretical studies (Finchman and Richardson, 1954; Haughton et al., 1974; Wallace and Carmichael, 1992; Holzheid and Grove, 2002; O’Neill and Mavrogenes, 2002) have demonstrated that, at relatively reducing conditions (i.e., below the FMQ buffer curve) and low pressure (i.e., consistent with chondrule formation), sulfur dissolves as sulfide (S2) in silicate melts by directly replacing oxygen on the anion sublattice according to:

bulk chondrules PO & PP zones

0.2

0

0

5

10

POP PO

15

abundance of sulfides (%) Fig. 5. Modal abundance of sulfides (%) reported versus modal abundance of low-Ca pyroxene estimated from the image processing of the EDX maps of 25 type I chondrules (bulk chondrules – filled circles) of all petrologic types (i.e., PO, POP & PP; Table 2) and of the different zones (PO & PP zones – grey squares) in four specific chondrules. The area (number of pixels) characteristic of each chondrule was determined by excluding the matrix and the rim on the basis of the iron and magnesium content of silicates. The area (number of pixels) of the different silicate phases (olivines, pyroxenes & mesostasis) and the metallic phases (Fe–Ni metal and sulfides) were determined by adjusting the histogram of the distribution of colors within the image. The modal abundances correspond to the ratio of the number of pixels for a given mineralogical phase to the total amount of pixels for the whole chondrule.

section of broken chondrules excluding parent body metamorphism and/or hydrothermal alteration, as elemental re-distribution would have resulted in sulfide being distributed on both sides of the broken chondrules (Fig. 6). Chondrules do not contain magnetite–carbide–sulfide association, which (Krot et al., 2004) used as evidence for hydrothermal alteration. Troilites associated with low-Ca pyroxenes occur in massive structures and not as thin veins that cross certain chondrules and which are the results of impact-induced transport (Rubin, 1992; Rubin et al., 1999) (Fig. 3). We found few occurrences where troilites are associated with Fe–Ni metal in OA structures, which rules out a secondary origin by sulfurization of metallic spherules either during cooling of chondrules or on the parent body itself (Blum et al., 1989; Lauretta et al., 1997a,b; Schrader and Lauretta, 2010). From a chemical perspective, the systematic FeO vs. S correlation (Fig. 7) is suggestive of high temperature igneous processes because it is similar to those observed in naturally occurring sulfur-rich basaltic magmas (Liu et al., 2007) and in sulfur solubility experiments in Fe-bearing silicate melts (O’Neill and Mavrogenes, 2002). Hence, these observations do not support a secondary origin of the chondrule sulfides but instead suggest a primary high-temperature origin contemporaneous with the events that formed the chondrules themselves (Hezel et al., 2010). This conclusion is consistent with the

2 S2 ðsilicate meltÞ þ 1=2O2 ¼ Oðsilicate meltÞ þ 1=2S2 :

ð1Þ

Because the abundance of O2 ions in silicate melts greatly exceeds the abundance of other anions including S2 (i.e., ln [O2]  0), reaction (1) suggests the following relationship: lnCs ¼ ln½S þ 1=2lnðf O2 =f S2 Þ

ð2Þ

where [S] is the sulfide content of the melt (in ppm) and Cs, which is analogous to the equilibrium constant for reaction (1), is called the ‘sulfide capacity’ of the melt (Finchman and Richardson, 1954). Using a large data set including synthetic and natural silicate melts, (O’Neill and Mavrogenes, 2002) developed a model for Cs of the form: ln Cs ¼ A0 þ RM X M AM

ð3Þ

where the XM term represents the mole fraction of cation M, the AM coefficients represent the preference of a metal for sulfur as a neighbor over oxygen and A0 is a constant combining the conversion factor between wt% and mole fraction with the activity coefficient for the sulfur species in the melt. Despite the wide range of compositions used, (O’Neill and Mavrogenes, 2002) concluded that S solubility in natural basaltic melt is predominantly governed by its FeO content since AFe >> ACa > AMg, ANa/K, ATi, (i.e., the higher the FeO content, the higher the sulfur solubility) (Li and Naldrett, 1994). The covariance of S and FeO in the type I chondrules mesostasis (Fig. 7) similar to that observed in basaltic liquids (Wallace and Carmichael, 1992; Jugo et al., 2005; Liu et al., 2007), including synthetic Febearing silicate melts (Haughton et al., 1974; Backnaes and Deubener, 2011), suggest that the sulfur content in chondrule mesostases is determined by the high temperature solubility law described above. An important feature of sulfur behavior on silicate melts is the existence of a concentration limit: the sulfur content at sulfide saturation (hereafter SCSS), at which a sulfur-rich

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Table 2 Modal abundances of the different phases characteristic of chondrules. All petrological types of chondrule are represented (PO, POP and PP). For 4 chondrules (Ch-25 and Ch-28 in Vigarano-P; Ch-06 and Ch-30 in Vigarano-477-2), the modal abundances have been determined for the inner part mainly made up of olivines (PO-zone) and the outer zone dominated by low-Ca pyroxenes (PP-zone). Meteorite

Chondrule

Type

Olivines (%)

Enstatites (%)

Sulfides (%)

Mesostasis (%)

Metal (%)

En/(En + Ol)

Vigarano-P

Ch-00 Ch-01 Ch-02 Ch-03 Ch-05 Ch-06 Ch-07 Ch-08 Ch-09 Ch-10 Ch-11 Ch-12 Ch-13 Ch-16 Ch-25

PO PO POP POP POP POP POP POP POP POP POP POP POP POP PO-zone PP-zone PO-zone PP-zone POP PP PP PP PO

79.66 71.64 35.95 61.96 31.75 25.51 69.05 30.62 31.97 53.91 50.93 21.03 34.57 39.17 87.69 6.07 93.81 4.16 52.56 5.35 7.03 3.78 82.36

8.020 12.17 42.45 18.83 40.48 52.24 16.93 37.36 41.80 33.72 32.69 47.24 40.23 40.66 10.27 85.59 0.23 84.00 28.48 73.17 69.98 74.55 6.31

1.04 0.67 9.96 5.09 5.96 11.61 3.71 7.06 8.77 5.18 7.36 8.27 11.91 5.07 0.46 6.69 0.79 10.30 6.35 12.07 12.19 11.92 2.46

6.37 10.36 7.97 9.18 11.74 7.62 5.95 16.01 7.49 3.18 4.89 16.29 8.19 8.32 1.36 0.74 3.26 0.82 7.64 2.36 2.91 2.99 2.17

4.91 5.16 3.66 4.95 10.08 3.03 4.35 8.95 9.97 4.00 4.13 7.17 5.10 6.78 0.23 0.91 1.91 0.71 4.97 7.05 7.89 6.76 6.70

0.0914 0.1452 0.5414 0.2331 0.5605 0.6719 0.1969 0.5495 0.5666 0.3848 0.3909 0.6920 0.5379 0.5090 0.1048 0.9338 0.0025 0.9528 0.3514 0.9365 0.9870 0.9517 0.0712

POP POP POP PO POP PO-zone PP-zone PP PO-zone PP-zone

50.31 62.50 40.62 81.69 21.64 90.31 1.35 4.65 83.82 8.15

29.36 17.91 35.39 6.31 57.18 4.71 86.22 68.19 0.58 74.73

3.03 2.35 9.74 2.17 7.84 0.15 10.40 15.98 0.54 6.17

8.47 2.35 6.67 4.39 9.05 4.10 1.76 6.45 12.10 6.79

8.83 14.89 7.58 5.44 4.29 0.72 0.27 4.73 2.97 4.16

0.3685 0.2227 0.4656 0.0717 0.7254 0.0496 0.9846 0.9362 0.0068 0.9017

Ch-28 Ch-33 Ch-44 Ch-45 Ch-46 Ch-52 Vigarano-477-2

Ch-01 Ch-02 Ch-03 Ch-04 Ch-05 Ch-06 Ch-09 Ch-30

condensed phase, most commonly an immiscible iron sulfide liquid at high temperature, separates from the silicate melt (Liu et al., 2007). The sulfur content in the silicate melt in equilibrium with this iron sulfide phase can be thus described by the following equation: FeOðsilicate meltÞ þ 1=2 S2ðgasÞ ¼ FeSðsulfideÞ þ 1=2 O2ðgasÞ

obtained by multiple linear regressions of experimental data and the best model found for the prediction of the SCSS was: 4454:6 P  0:03190 þ 0:71006 T T  lnðMFMÞ  1:98063½ðMFMÞðX H2 Omelt Þ

ln ½SSCSS ¼ 11:35251 

ð4Þ

þ 0:21867 lnðX H2 Omelt Þ þ 0:36192

for which the equilibrium constant can be written:

 lnðX FeOmelt Þ



DGð4Þ =RT ¼ lnaFeSðsulfideÞ  lnaFeOðsilicate meltÞ þ 1=2lnðf O2 =f S2 Þ

ð5Þ

Using such thermodynamic approaches, several empirical expressions have been proposed that allow the sulfur content at sulfide saturation for any silicate melt composition to be calculated (O’Neill and Mavrogenes, 2002; Liu et al., 2007; Li and Ripley, 2009). Among them, we used the most recent ones to calculate the sulfur content at sulfide saturation in PO and PP chondrules (Liu et al., 2007; Li and Ripley, 2009). In the expression of (Liu et al., 2007) [hereafter, LSB2007], the coefficients proposed were

ð6Þ

where S is in ppm, P is in bar, T is in Kelvin, MFM is a compositional parameter describing the melt based upon cation mole fractions: MFM ¼

Na þ K þ 2  ðCa þ Mg þ Fe2þ Þ ; Si  ðAl þ Fe3þ Þ

X H2 Omelt is the mole fraction of water in the melt, and X FeOmelt is the mole fraction of FeO in the melt. Following the same approach, (Li and Ripley, 2009) [hereafter, LR2009] proposed an other expression for the sulfur content at sulfide saturation in silicate melts:

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Fig. 6. (a and d) Back-scattered electron images of two broken chondrules (Ch-13 & Ch-33; Vigarano-P). (b and e) EXD map of sulfide (purple) distribution in the two broken chondrules. (c) & (f) Compiled EDX maps of Si and Mg of the two PP chondrules revealing the mineralogy of the different chondrule silicate phases: olivines (orange), low-Ca pyroxene (pale green) and mesostasis (dark green). As for intact chondrules, sulfides are also associated with low-Ca pyroxenes on the convex part of the chondrules. The absence of sulfides on the broken edge of the chondrules suggests that secondary processes on the parent body are unlikely to be the origin of the sulfide distribution.

 4 10  0:021P þ 5:559X FeO ln X S ¼ 1:76  0:474 T þ 2:565X TiO2 þ 2:709X CaO  3:192X SiO2  3:049X H2 O

ð7Þ

where P is in kbar, T is in Kelvin, and X is the mole fraction. The sulfur content at sulfide saturation in chondrules was calculated using the average mesostasis composition of chondrule #4 of Vigarano 477-2 (Table 3) as a proxy of type I PO compositions, while the average type I PP chondrule mesostasis composition of Semarkona (Jones, 1996) was used for modeling PP chondrules due to the difficulty finding non-devitrified glass in POP/PP chondrules of Vigarano (Table 4). Sulfide saturation (Fig. 8) was calculated with the above three expressions at 1500 °C for each

composition, in order to match the low-Ca pyroxene saturation field in POP and PP chondrules (i.e., <1557 °C). It is important to note that the [S]SCSS for chondrules calculated with the expressions of LBS2007 and LR2009 correlate positively with the FeO content of the mesostasis (Fig. 8). In addition, the temperature effect on [S]SCSS using the two models is rather limited, varying by just a few hundred ppm in the range 1400–1600 °C. The [S]SCSS calculations performed using the two models previously described shows that sulfide saturation (i.e., around [S]SCSS  1500 ppm) is not reached in the mesostasis of PO chondrules (Fig. 8a), consistent with the lack of sulfide phases in these chondrules (Fig. 5). In contrast, the FeS saturation in PP chondrule mesostasis drops significantly to [S]SCSS values lower than 500 ppm (Fig. 8b) in response to the change in melt composition, and notably the increase in bulk polymerization of the melt from PO to PP,

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Table 3 Chemical composition of glassy mesostasis in five different chondrules from Vigarano. Chondrule

Na2O

SiO2

AI2O3

MgO

SO3

FeO

K2O

CaO

P2O5

TiO2

Sum

S

Ch-52

0.48 0.46 0.52 0.54 0.44 0.62 0.61 0.70 0.72 0.23 0.84 0.53 0.78 0.62 0.69 0.75 0.50

41.73 43.97 44.42 43.47 47.24 44.62 45.84 43.98 44.06 47.04 43.91 42.50 43.56 42.45 43.45 43.18 45.68

28.86 30.20 33.20 32.75 25.08 32.96 28.29 32.01 30.95 21.69 33.16 32.70 32.71 30.76 32.12 32.04 30.13

9.55 5.30 1.69 2.30 7.37 1.76 4.78 3.02 5.27 9.65 0.79 3.75 1.84 3.93 2.76 2.28 3.35

0.16 0.18 0.12 0.23 0.09 0.11 0.11 0.11 0.13 0.12 0.20 0.17 0.16 0.25 0.16 0.28 0.12

1.84 1.24 0.72 1.68 0.63 0.76 1.46 1.29 1.60 1.56 2.45 2.09 1.38 3.21 1.98 2.95 0.69

bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl

16.06 17.74 18.36 18.14 18.89 18.61 17.76 17.77 16.44 18.57 17.79 17.50 17.83 17.29 17.78 17.03 18.54

bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl

0.76 0.63 0.59 0.98 0.21 0.34 0.52 0.97 0.42 0.68 0.30 0.41 0.79 0.67 0.27 0.61 0.33

99.43 99.70 99.60 100.08 99.94 99.79 99.38 99.85 99.59 99.55 99.44 99.66 99.04 99.19 99.19 99.11 99.33

0.06 0.07 0.05 0.09 0.03 0.04 0.04 0.04 0.05 0.05 0.08 0.07 0.06 0.10 0.06 0.11 0.05

Ch-12

3.60 2.75 1.96 1.97 1.83 1.36 1.28 1.50 2.81

44.10 43.53 43.77 45.65 45.78 44.49 48.13 48.07 43.46

33.04 28.58 27.47 28.32 27.28 32.01 22.90 26.12 32.39

1.35 6.48 6.48 5.13 7.94 1.95 8.47 9.23 2.21

0.10 0.25 0.42 0.10 0.17 0.28 0.09 0.07 0.14

1.58 2.29 3.51 0.77 2.06 1.86 1.32 1.38 1.94

0.10 bdl bdl bdl bdl bdl 0.07 bdl 0.07

15.40 14.41 14.21 16.55 14.02 17.19 15.65 12.66 16.10

bdl bdl 0.09 0.06 bdl bdl bdl bdl bdl

0.27 0.49 0.99 0.71 0.34 0.29 0.81 0.39 0.19

99.55 98.77 98.90 99.26 99.41 99.43 98.72 99.43 99.30

0.04 0.10 0.17 0.04 0.07 0.11 0.03 0.03 0.06

Ch-33

1.06 1.35 1.13 0.92 1.02 1.26 1.70 1.26 1.11 1.32 1.53 1.29 1.19 1.14 1.12 1.13

47.33 47.30 47.59 47.31 47.70 48.16 47.24 47.76 47.58 48.31 48.43 48.98 48.80 48.50 47.34 47.68

27.48 26.15 25.93 26.10 24.96 25.37 27.49 25.07 25.37 25.08 25.02 24.18 25.41 25.88 26.41 26.28

5.26 8.25 6.76 7.29 7.42 7.28 5.36 6.72 6.95 6.99 6.89 7.10 6.97 7.09 7.46 6.96

0.08 0.12 0.08 0.08 0.18 0.13 0.20 0.30 0.13 0.26 0.16 0.15 0.07 0.21 0.09 0.12

0.66 0.65 0.66 0.54 0.78 0.64 0.77 1.10 0.82 0.93 0.77 0.60 0.41 0.73 0.50 0.62

bdl 0.07 bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl

16.38 15.58 16.52 17.02 16.55 16.34 15.36 16.35 16.61 16.68 16.41 16.80 16.37 16.05 15.39 16.21

bdl 0.10 bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl 0.04 bdl bdl bdl

0.31 0.57 0.42 0.71 0.97 0.42 1.07 0.87 0.78 0.24 0.37 0.65 0.54 0.26 0.93 0.68

98.57 100.14 99.08 99.96 99.57 99.59 99.20 99.42 99.36 99.81 99.59 99.75 99.79 99.86 99.23 99.67

0.03 0.05 0.03 0.03 0.07 0.05 0.08 0.12 0.05 0.10 0.06 0.06 0.03 0.08 0.04 0.05

Ch-16

1.51 1.08 1.02 1.67 1.72 1.51 1.47 1.14

47.63 46.58 48.27 47.12 49.94 47.58 47.64 47.39

24.16 25.30 22.94 24.76 22.42 24.23 24.30 24.65

8.25 6.93 7.52 6.87 6.86 7.23 7.35 7.24

0.33 0.20 0.07 0.24 0.19 0.09 0.06 0.10

1.52 1.51 0.62 1.63 0.88 0.57 0.65 0.97

bdl bdl bdl bdl bdl 0.09 bdl bdl

15.18 16.75 18.22 16.71 17.01 16.76 16.86 17.26

0.11 0.09 bdl bdl 0.08 bdl bdl bdl

0.85 0.73 0.98 0.56 0.36 1.16 0.91 0.85

99.53 99.16 99.63 99.56 99.46 99.22 99.24 99.60

0.13 0.08 0.03 0.10 0.07 0.04 0.02 0.04

Ch-04

1.22 1.26 1.24 2.33 3.70 2.49 1.18 1.11 1.15

47.86 47.61 47.27 47.13 47.16 47.83 47.83 46.65 47.86

24.60 23.72 24.00 25.37 24.87 23.53 24.76 27.20 25.23

7.44 8.02 6.82 6.96 7.13 8.33 7.17 6.73 6.58

0.10 0.19 0.15 0.17 0.28 0.15 0.09 0.06 0.13

1.07 1.47 0.79 0.76 1.54 0.72 0.60 0.39 0.57

bdl bdl bdl bdl bdl bdl bdl bdl bdl

16.03 16.36 17.10 15.77 14.39 15.68 17.20 17.45 16.90

bdl bdl bdl bdl bdl bdl bdl bdl bdl

1.01 99.33 0.77 99.38 0.71 98.08 1.09 99.58 0.66 99.72 0.76 99.48 0.49 99.31 0.52 100.10 0.86 99.29 (continued on next

0.04 0.07 0.06 0.07 0.11 0.06 0.03 0.02 0.05 page)

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Table 3 (continued) Chondrule

Na2O

SiO2

AI2O3

MgO

SO3

FeO

K2O

CaO

P2O5

TiO2

Sum

S

6.68 1.14 1.00 2.25 1.02 1.47 4.93

44.84 47.40 48.04 47.35 49.64 48.18 48.86

22.34 24.03 24.61 24.32 23.14 24.42 21.45

8.71 7.32 7.19 7.31 7.03 7.05 7.57

0.39 0.26 0.16 0.21 0.14 0.07 0.38

2.30 0.91 0.69 0.90 0.71 0.72 1.33

0.03 bdl bdl bdl bdl bdl 0.10

13.20 17.04 17.18 16.00 16.56 17.06 14.06

bdl bdl bdl bdl bdl bdl bdl

0.96 1.17 0.42 0.29 0.38 0.72 0.65

99.46 99.27 99.29 98.62 98.61 99.69 99.33

0.16 0.10 0.06 0.08 0.06 0.03 0.15

Fig. 7. Concentration of FeO wt% and sulfur (expressed as wt% of S) measured by electron microprobe in different zones of well-preserved glassy mesostasis in four PO and POP chondrules (Ch-52, Ch-12, Ch-33 in Vigarano-P and Ch-04 in Vigarano-477-2). The sulfur concentration detection limit has been estimated to be 75 ppm. The observed positive correlations between the iron and sulfur content of the mesostases are consistent with high temperature solubility of sulfur (see text for explanations).

all other parameters being constant (i.e., temperature and FeO melt content). In light of these results, two main factors control the sulfur solubility in silicate melts: first, the FeO content of the silicate melt and then, when FeS saturation is reached, the polymerization state of the liquid; the [S]SCSS decreasing with increasing polymerization of the liquid (O’Neill and Mavrogenes, 2002). The positive

correlation between S and FeO content in chondrules (Fig. 7) and the close association of troilites and low-Ca pyroxenes in POP and PP chondrules (Fig. 5) can be thus interpreted within this framework. Petrographic (Hezel et al., 2003, 2006; Krot et al., 2004; Libourel and Krot, 2007) and experimental (Tissandier et al., 2002) observations suggest that gas–melt interactions

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Table 4 Chemical composition (wt%) of phases used as proxies to address the question of the carrier of S and Fe in bulk chondrules via mass balance calculations (see text). Olivine, low-Ca pyroxene and PP mesostasis data are from (Jones and Scott, 1989; Jones, 1996).

SiO2 TiO2 Al2O3 Cr2O3 FeO MgO CaO MnO Na2O K2O Fe Ni Co S

Olivines

Low-Ca pyroxenes

PO mesostasis

PP mesostasis

Fe metal

FeS sulfides

42.13 – – 0.37 0.96 55.65 0.31 – – – – – – –

58.58 0.36 0.39 0.63 2.10 37.61 0.27 0.28 – – – – – –

49.90 0.97 24.30 0.43 0.43 5.39 18.10 0.06 0.32 0.10 – – – –

66.90 0.36 15.40 0.33 4.20 0.33 2.60 – 6.70 0.93 – – – –

– – – – – – – – – – 93.90 4.45 0.41 –

– – – – – – – – – – 63.00 – – 37.00

may have played a major role during the formation of type I chondrules (Libourel et al., 2006). In this scenario, chondrule melt is considered to be an open system and its composition, mainly the SiO2 content, is controlled by exchange with the surrounding gas (e.g., SiO gas partial pressure), the dissolution of precursor olivines (Kropf and Libourel, 2011; Soulie´ et al., 2012) and the crystallization of low-Ca pyroxenes (Tissandier et al., 2002); the nature of the type I chondrules (PO, POP, PP) being dependent on the duration of such gas–melt interactions (heating). During this process, sulfur can be dissolved into the chondrule melt (Eq. (1)) and may reach the sulfide saturation (Eq. (4)) depending on the fS2 of the surrounding nebular gas (Fig. 8a). At fixed fS2, the sulfur content at sulfide saturation decreases as the melt evolves towards a more silicic composition due to the incoming silica from the surrounding gas (high PSiO(gas)). This may eventually lead to the formation of iron sulfides (Fig. 8b) that will co-crystallize with low-Ca pyroxenes, in agreement with occurrences of sulfide-bearing inclusions embedded within low-Ca pyroxenes (Figs. 1 and 2) and the positive correlation between the abundance of low-Ca pyroxene and sulfide globule (Fig. 5). Since this model for chondrule formation involves gradual addition of SiO2 (Na2O and FeO, see below), the melt composition and the activity coefficient of FeO vary widely throughout this gas–melt interaction process, and must affect the S solubility. Because PO and PP mesostasis compositions are the two end members of this gas–melt interaction, comparison of S solubility in PO (Fig. 8a) and PP (Fig. 8b) mesostasis provides a simple way to evaluate the effect of these compositional covariations on the S solubility. Qualitatively, each SCSS model (O’Neill and Mavrogenes, 2002; Liu et al., 2007; Li and Ripley, 2009) than take into account the effect of melt composition on the sulfur content at sulfide saturation confirms that SiO2 and FeO have the more prominent effect. An increase in SiO2 (or the increase in bulk melt polymerization as measured by the ratio between network forming cations: Si, Al, and network modifying cations: Ca, Mg, Fe, Na, K) results in a decrease of [S]SCSS in the mesostasis. In contrast, the addition of FeO will favor an increase in [S]SCSS. Quantitatively, the gradual addition of SiO2,

Na2O and FeO to reach PP mesostasis composition from PO ones results in a decrease in [S]SCSS of around  1000 ppm (Fig. 8). This suggests that, as a general rule, both the silica (i.e., the polymerization) and the iron content are the main controlling factors for sulfide saturation in the chondrule melts. Since SiO2 enters chondrules from the gas phase only at the edge of the chondrule, this process may explain both the core (PO) to rim (PP) positive correlation between the modal abundance of low-Ca pyroxene and troilite that we observed in POP chondrules, and the gradual modal changes between PO, POP and PP chondrules (Fig. 5). 4.3. Iron sulfide wetting properties in chondrules The formation of troilites from the crystallization of FeS-rich immiscible liquid due to sulfur saturation of the chondrule melt is further supported by the study of the morphology and the wetting angles of sulfide globules in contact with olivines/low-Ca pyroxenes and glassy silicate mesostasis (Fig. 3c, d). Textural equilibrium between solids and liquids under a hydrostatic state of stress is determined by the relative values of the interfacial energies. From mechanical and thermodynamic considerations, the equilibrium geometry is the one that minimizes the total interfacial free energy for all interfaces in the system by balancing the surface tensions at grain–grain and grain–liquid intersections in order to eliminate chemical potential gradients (Bulau et al., 1979). This criterion is expressed by the wetting angle equation, which relates equilibrium interfacial angles to interfacial energies. In a three-phase system (Fig. 9), solid-plus-two immiscible melts, the wetting angle w between the two melts interface and the solid substrate is given by Young’s equation: 0 ¼ csl1  csl2  cl1l2 cosðwÞ

ð8Þ

In this case, the wetting angle is determined by the csl1 solid–liquid1, csl2 solid–liquid2, and cl1l2 liquid1–liquid2 interfacial energies (Mungall and Su, 2005). The liquid for which w is <90° said to wet the solid and is termed the wetting liquid while when w is >90° the liquid is termed nonwetting.

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Fig. 8. Sulfur content at FeS sulfide saturation (SCSS; in wt% of S) at 1500 °C for representative mesostasis compositions of PO (a) and PP (b) chondrules as a function of the FeO content (in wt%) of the melt using the thermodynamic expressions of Liu et al. (2007) (dotted line) and Li and Ripley (2009) (dashed line). The SCSS corresponds to the saturation limit at which an immiscible sulfide liquid will separate from the silicate melt. The filled circles represent the chemical compositions (in wt%) of the glassy mesostases of the five chondrules measured in this work (Ch-04 from Viragano-477-2 and Ch-12, Ch-16, Ch33 and Ch-52 from Vigarano-P). (a) These two models show that the sulfide saturation is not reached in PO chondrules, as suggested by the lack of troilite in these chondrules. (b) In contrast, the SCSS calculated for the silica-rich PP mesostasis is lower, favoring troilite saturation at much lower S content. This is supported by the higher abundance of sulfides in PP chondrules relative to PO ones.

Measurements of interfacial tension csb between iron sulfide (s) and basaltic silicate (b) melts (l1 and l2 in Eq. (8)) range from about 0.5 to 0.65 J m2 (Mungall and Su, 2005), values that are higher than those of the interfaces between silicate minerals and silicate melt. For instance, (Wanamaker and Kohlstedt, 1991) determined the cbo interfacial tension between olivine (o) and various synthetic melts to be less than 0.5 J m2; these data are consistent with previous measurements performed by Cooper and Kohlstedt (1984) on San Carlos olivines and basaltic melt. Using Young’s equation (Eq. (8)) with the above values for csb and cbo and a value of cso = 0.71 J m2 as recommended by Holzheid et al. (2000), a contact angle w between the iron sulfide–silicate melt interface and the

olivine can be estimated. In the present case, the contact angle w must be larger to 115–130°, suggesting that iron sulfide liquid is non-wetting in contact with olivine crystals in the presence of silicate melt (and by extension with low-Ca pyroxenes), and that the sulfide melt will form discrete droplets. The typical spheroidal or ameboidal shapes of troilites within chondrules (Fig. 3) thus very likely result from the high interfacial tension between their precursor iron-sulfide liquid droplets and the surrounding silicate melt during the high temperature chondrule-forming event (Rubin et al., 1999). Due to this non-wetting behavior, there will be a thin film of silicate melt between the sulfide droplets and olivines. Where these droplets are in contact with olivines, they will wet them very little or not at all, given that their contact angles w are expected to be greater than 115–130° from the above calculation and very likely on the order of 140–170° by analogy with the experiments of (Mungall and Su, 2005). Droplets with such high contact angles will not be able to penetrate openings that are already there, nor generate new apertures at olivine–olivine grain boundaries by capillary forces. Such predicted configurations match those observed for large ameboidal sulfide pools in Vigarano chondrules (Fig. 3c, d), in which contact angles w between the interfaces of the two melts and the olivine are obtuse and on the order of 160–170° (Fig. 3c). This feature provides convincing evidence in favor of the occurrence of two immiscible melts during chondrule formation, a silicate melt and an iron sulfide melt, since any post-silicate deposition of sulfides that occurs below the glass transition temperature (e.g., during parent body processes) will be unable to produce these wetting textures. 4.4. Implications for the formation of chondrules From the above, it appears that sulfide droplets in type I chondrules: (i) have wetting properties similar to those of iron sulfide liquid in contact with a silicate melt, (ii) obey to sulfide saturation laws in silicate melts and (iii) have modal abundances that are positively correlated with a high temperature crystallizing phase (i.e., low-Ca pyroxene). These features cannot be simply fortuitous and suggest that sulfide pockets in chondrules were very likely formed from the crystallization of FeS-rich immiscible liquid following sulfur saturation of the chondrule silicate melt. Due to the volatile behavior of sulfur under nebular canonical conditions, the present findings have profound implications on chondrule forming events and are summarized in the following. 4.4.1. Phase relationship in chondrules Sulfide droplets in type I chondrules occurring exclusively as inclusions in low-Ca pyroxenes (not in olivines) and in mesostasis indicate that sulfide saturation is a high temperature process. The positive correlation between the abundance of sulfide and low-Ca pyroxene (Fig. 5) in a given chondrule or from chondrule to chondrule indicates that such FeS saturation is linked to a drastic increase in the silica activity of the melt (a SiO2), very likely in response to interaction with high PSiO(gas) of the surrounding

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Fig. 9. Schematic view of the wetting angle w in a three-phase system between the interface of two melts and a solid substrate. A liquid for which w < 90° (liquid 2, left) is said to wet the solid and is called the wetting liquid while when w > 90° (liquid 2, right), the liquid is called nonwetting. According to literature data (Cooper and Kohlstedt, 1984; Wanamaker and Kohlstedt, 1991; Holzheid et al., 2000), iron liquids are not expected to wet minerals (i.e., olivines and low-Ca pyroxenes) in the presence of silicate melts, and consequently sulfide melt should form spheroidal and/or ameboidal droplets. These results are consistent with the observed distribution and morphology observed within chondrules (Fig. 3c, d) and confirm that troilites originate from the crystallization of FeS-rich immiscible liquid due to sulfur saturation of the chondrule melt.

environment during chondrule formation events. According to the proposed reaction: Mg2 SiO4ðolivineÞ þ FeOðmeltÞ þ SiO2ðmeltÞ þ 1=2S2ðgasÞ ¼ Mg2 Si2 O6ðpyroxeneÞ þ FeSðsulfideÞ þ 1=2O2ðgasÞ

ð9Þ

such an increase in silica activity in the chondrule melt, for a given sulfur fugacity in the surrounding gas, will be responsible for: (i) olivine dissolution and then (ii) co-crystallization of low-Ca pyroxene and FeS. Because the majority of iron sulfide globules are not associated with metal in OAs (Fig. 3), sulfides trapped or associated with low-Ca pyroxenes are unlikely to originate from high temperature sulfurization of Fe–Ni metal blebs in the chondrules, as summarized by the following reaction: Mg2 SiO4ðolivineÞ þ FeðmetalÞ þ SiO2ðmeltÞ þ 1=2S2ðgasÞ ¼ Mg2 Si2 O6ðpyroxeneÞ þ FeSðsulfideÞ

ð10Þ

As a consequence, the iron in these sulfides must come from other sources, very likely from the chondrule mesostasis (Eq. (9)) and/or alternatively from the gas phase in the event where the FeO content of the molten mesostasis is partially buffered by the Fe partial pressure of the surrounding gas (see below). According to our textural observations, the sequence of events in type I chondrules should be as follows: (i) olivine and metal blebs are precursor grains; (ii) due to the entry of silica in the mesostasis, olivine is no longer stable and is dissolved into the mesostasis, favoring crystallization of lowCa pyroxene; (iii) at the same time, sulfide saturation occurs in the melt and FeS starts to crystallize in the mesostasis or in close association with low-Ca-pyroxene; (iv) sulfide crystallization may eventually continue after low Ca-pyroxene crystallization as shown by the frequent trapping of lowCa pyroxenes in sulfide pools (Fig. 3a). While a full discussion is beyond the scope of this paper, it is very likely that the Fe–Ni metal blebs could have been sulfidized to form the so-called opaque assemblages by: (i) interaction with gaseous sulfur (Schrader and Lauretta, 2010) and/or (ii) interactions with immiscible sulfide droplets (Fig. 4). From contact angle observations, these sulfide pockets are necessarily molten and act as non-wetting melts on olivine/lowCa pyroxene grains in the presence of the silicate mesostase

melt (Fig. 3c, d). The preservation of these wettability textures suggests that the chondrule heating event was terminated by rapid cooling well above the glass transition temperature, which is on the order of 1000–1050 K for a PP-like mesostasis composition. 4.4.2. Conditions of formation From their survey of ordinary chondrites, (Rubin et al., 1999) suggested that the presence of primary sulfides in type I chondrules requires: (i) nebular temperatures lower than 650 K at the time of chondrule formation and (ii) very short periods of melting (i.e., a few tenths of a second) in order to avoid significant volatilization of S. (Tachibana and Huss, 2005) confirmed this view by showing that the primary troilites present no significant sulfur isotopic fractionations (i.e., <1&/amu) relative to large troilite grains in the matrix. They proposed that a heating rate of  104–106 K/h would be required to avoid a large degree of sulfur isotopic fractionation in the chondrule precursors. In addition, assuming that the main opaque phases in type I chondrules are metallic Fe–Ni and sulfide, (Rubin, 2010) estimated the proportion of type-I chondrules that contain sulfides in their interiors to be 47% in Vigarano, and highly variable from chondrite to chondrite (e.g., more than 80% in OC3, around 50% in CV, close to 3% in CR and 0% in CK). These variations are interpreted as the results of preferential sulfur evaporation from the chondrule surface after rapid migration of metal and sulfide from the interior to the surface due to wicking and interface tension within the chondrules (Rubin, 2010). However, the lack of sulfide inclusions in olivines on the one hand (i.e., consistent with the absence of sulfides within CV-chondrule precursors) (Zanda et al., 2009) and the compelling correlation between low-Ca pyroxene and iron sulfide modal abundances on the other (Fig. 5) are not consistent with the above model (Rubin, 2010). This conclusion is supported by the calculations performed by Uesugi et al. (2005), which show that large iron sulfide inclusions would reach the surface of chondrules and evaporate within a short period of time (<<1 s). Alternatively, the increase in sulfide modal abundances towards the surfaces of chondrules is consistent with a model in which sulfur solubility in chondrule mesostasis and sulfide saturation processes are imposed by the partial pressures

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chondrules remains, however, an open question, even if impact-generated plumes are plausible environments (Fedkin and Grossman, 2013).

Fig. 10. Log fO2 vs log fS2 diagram. Plausible range of fO2 and fS2 calculated from Eq. (2) at 1500 °C for the formation of low-Ca pyroxene bearing POP/PP chondrules as a function of the FeO content of the mesostasis at FeS saturation (solid lines for 0.5, 1, 2 3 5 wt% FeO) for a representative type I PP chondrule mesostasis composition (see Table 4). Occurrence of iron sulfide liquids within PP chondrules requires an oxygen fugacity between log fO2 > 13.5 (>IW-3) and a sulfur fugacity between log fS2 > 3.4. These results show that the formation of chondrules took place in volatile-rich environments with non canonical conditions for both oxygen (canonical  IW-7) and sulfur fugacities.

reigning in the gaseous environment during chondrule formation (Eq. (9) and see below). The determination of a plausible range of fO2 and fS2 for the formation of low-Ca pyroxene bearing POP/PP chondrules as a function of the FeO content of the mesostasis is possible by solving Eq. (2) at FeS saturation for the average type I PP chondrule mesostasis composition at 1500 °C (Fig. 10). According to this calculation, for 1– 3 wt% FeO in the melt (i.e., the typical FeO content of PP melt in Vigarano), the co-saturation of low-Ca pyroxene and FeS at 1500 °C will be achieved at an oxygen fugacity between 13.5 < log fO2 < 9 (i.e., IW-3 to IW+1; oxygen fugacities between 3 log units below and 1 log unit above the iron-wu¨stite buffer, where metallic iron is in equilibrium with pure FeO) if one arbitrarily fixes the maximum sulfur fugacity at log fS2 = 0 (i.e., sulfur fugacity superior to Fe– FeS iron–iron sulfide sulfur buffer). These estimates are in good agreement with the oxygen fugacity proposed by Grossman (2010), Fedkin et al. (2012) for the formation of type I chondrules based on the FeO content of chondrule olivines (between IW-1 and IW-2). The occurrence of reactions between iron metal and iron sulfide blebs in POP/PP chondrules (Fig. 4) also suggests chondrule formation at higher sulfur fugacity than those imposed by the Fe–FeS sulfur buffer. While more work needs to be done to quantify the redox conditions (fO2 and fS2), these results point towards volatile-rich environments at the time of chondrule formation with conditions that are non canonical for both oxygen (the canonical is  IW-7) and sulfur fugacities. How to make systems sufficiently rich in oxygen and sulfur to produce these sulfide-low-Ca pyroxene assemblages in

4.4.3. Closed vs open system during chondrule formation In the last decade, several studies have been dedicated to the behavior of other volatile elements (mainly alkalis) because their abundance in chondrules can place severe constraints on their conditions of formation (Ebel and Grossman, 2000; Libourel et al., 2003; Alexander and Grossman, 2005; Alexander et al., 2008; Borisov et al., 2008). Most of these studies predicted non canonical environments during chondrule formation and (Alexander et al., 2008; Alexander and Ebel, 2012; Fedkin et al., 2012) have recently estimated that very high dust densities with solid/gas enrichments in excess of 105–106 are required in the nebula in order for chondrules to behave as essentially closed systems for Na. Because such closed system behavior for Na can be challenged for type I chondrules (Libourel et al., 2003, 2006), this may draw into question whether this very high degree of dust/gas enrichment during chondrule formation is correct (e.g., equivalent to number densities of millimeter-sized chondrules larger than 103– 104 m3). In comparison with alkalis, S is a more suitable indicator of evaporation or condensation processes for two main reasons. First, with an equilibrium 50% condensation temperature of 664 K for a solar gas composition at 104 bar (Lodders, 2003), sulfur is the most volatile major element in chondrites and therefore evaporates more rapidly at any given high temperature. Second, sulfides are forced to precipitate from the melts at low sulfur contents (see above). In the absence of measurable S isotopic mass fractionation in troilite grains of FeO-poor chondrules (Tachibana and Huss, 2005), the present findings show that sulfur solubility and sulfide saturation in chondrule mesostasis are driven by interaction (i.e., condensation) with the gas phase and at the ambient fO2 and fS2 no sulfur evaporation is predicted. The systematic positive correlation between S and Na (Nagahara et al., 2008) in Vigarano type I chondrules (Table 3) suggests that such a process of gas–melt interaction could also be viable for less volatile elements such as alkalis. This is consistent with both the concentric zoning in Na, with enrichments near the outer margins (Grossman et al., 2002; Nagahara et al., 2008) and the lack of significant potassium isotopic mass fractionation (Alexander et al., 2000). As shown in Figs. 1 and 2, the high abundance of sulfides at the edges of type I chondrules in close association with the low-Ca pyroxene raises the question of the carrier of S and Fe in the bulk chondrules. A simple mass balance calculation by weighted modal recombination analysis of the phase compositions from (Jones and Scott, 1989; Jones, 1996) used as phase proxies (Table 4) and modal abundances given in Table 2 allows us to tackle this issue. From PO to PP, the Fe fraction from sulfides increases by almost an order of magnitude while at the same time the Fe contribution from metal and silicate phases remains constant (Fig. 11). This suggests that up to 40–50% of total Fe by mass comes from sulfides for the PP Vigarano bulk chondrule composition. These PO–PP modal changes may also

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Fig. 12. CI Normalized Fe (grey squares), Na (open circles) and S (filled circles) composition of bulk type I Vigarano chondrules. It is suggested that the chondritic signature observed in PP chondrules is not inherited from the chondrule precursors but generated by interactions with the gaseous environments during chondrule formation (see text for explanation). Similar trends were observed between chondrules or inside single chondrules from the PO core to the PP rim (filled diamond for sulfur).

Fig. 11. Iron mass balance calculations and Fe distribution in bulk type I chondrules of Vigarano by weighted modal recombination analysis of modal abundances of phases determined in this study (Table 2). The iron fraction from sulfides (filled dots) increases from PO to PP while the iron contribution from metal (open circles) and silicate phases (grey circles) remain sub-constant. Up to 50% of total iron is estimated to come from sulfides for PP chondrules. Note that similar features are observed in a chondrule (filled diamond only for sulfides) from the PP rim to the PO interior (Table 2).

be observable in certain zoned chondrules (Fig. 1). Thus, according to the following expression, about one half of the total Fe that enters the chondrule comes from the gas phase (see also Nagahara et al., 2008): Mg2 SiO4ðolivineÞ þ ½FeNiðmetalÞ  þ FeðgasÞ þ SiO2ðmeltÞ þ 1=2S2ðgasÞ ¼ Mg2 Si2 O6ðpyroxeneÞ þ ½FeNiðmetalÞ  þ FeSðsulfideÞ ð11Þ which allows Mg-rich olivine and Fe–Ni metal blebs to be preserved in POP and PP chondrules as relict grains. Finally, by normalizing these abundances (Table 2) to the chondritic value (Fig. 12), one comes to the conclusion that the chondritic signature observed in PP chondrules is

not inherited from the chondrule precursors but is generated from interactions with the gaseous environments during chondrule formation (Eq. (10)). In contrast with (Hewins, 1997), enrichments in volatile to moderately volatile elements such as S, Na (and in part Fe and Si) from PO to PP type I bulk chondrule composition (see also (Lauretta et al., 2006)) result from the extent of reaction between partially depleted olivine-bearing precursors and a volatile-rich gas phase, in which the solubility in the mesostasis and/or the attainment of a given phase saturation in the mesostasis acts as a buffer, i.e., depending on the sulfur capacity (Cs) of the mesostasis and the ambient fO2 and fS2 in the present case of sulfur. 5. CONCLUSION Our survey of the nature and distribution of sulfides within type I PO, POP and PP chondrules of the carbonaceous chondrite Vigarano (CV3) reveals that: (i) S content in the chondrule mesostasis is positively correlated with FeO and (ii) troilites are mainly located within the lowCa pyroxene outer zone and their amounts increase with the abundance of low-Ca pyroxene within chondrules, suggesting the co-crystallization of troilite and low-Ca pyroxene at high-temperature during chondrule formation events. Thermodynamic calculations show that sulfur solubility and the attainment of sulfide saturation are very sensitive to the composition of the chondrule mesostasis and in particular the FeO and SiO2 content. Depending on the fS2 and fO2 of the surrounding gas and on the melt composition, sulfur dissolves in chondrule melts and may reach a concentration limit at which an immiscible iron sulfide liquid separates from the silicate melt. We also show that the occurrence of both a silicate melt and an immiscible

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iron sulfide liquid can be further supported by the nonwetting behavior of sulfides on silicate phases in chondrules. The evolution of chondrule melts from PO to PP towards more silicic compositions, very likely due to high PSiO(g) of the surrounding nebular gas, induces saturation of FeS at much lower S content in PP than in PO chondrules, leading to the co-crystallization of iron sulfides and low-Ca pyroxenes and explaining the systematic positive correlation between the modal abundance of low-Ca pyroxene and troilite. Conditions of co-saturation of low-Ca pyroxene and FeS are only achieved in non canonical environments characterized by high partial pressures of sulfur and SiO and redox conditions more oxidizing than IW-3. Fe and S mass balance calculations also suggest the presence of an external source of iron, very likely gaseous, during chondrule formation. We therefore propose that enrichments in sulfur (and other volatile and moderately volatile elements) from PO to PP type I bulk chondrule compositions towards chondritic values result from progressive reaction between partially depleted olivine-bearing precursors and a volatile-rich gas phase. Since sulfides record both high temperature processes during chondrule formation as well as low temperature processes during accretion and parent-body formation, studies of sulfides will provide one of the best probes for understanding the formation of chondrules and their secondary evolution. If it is generally accepted that reduced CV, including Vigarano, have to a certain extent escaped secondary alteration (Krot et al., 1995; Abreu and Brearley, 2011) it would be interesting to extend this kind of study to other CVs and carbonaceous chondrites. CR chondrites (Krot et al., 2002) for instance might challenge this present approach as the vast majority of sulfides are present in the matrix and not within chondrules while the bulk abundance of sulfur is the same as for CV and ordinary chondrites. The scenario presented here would predict that sulfur contents in the mesotasis of CR type I chondrules would have had to remain below the sulfur content at sulfide saturation (SCSS) and that sulfide saturation was not reached during the high temperature CR chondrule forming event. ACKNOWLEDGMENTS Michel Fialin and Fre´de´ric Couffignal are thanked for assistance during sample preparation and analysis. The authors are grateful to Laurent Tissandier, Francßois Faure, Pete Burnard, Romain Matthieu and Laurette Piani at CRPG and Sasha Krot, Garry Huss and Ed Scott at HIGP for helpful discussions. This study was supported by grants from the Programme National de Plane´tologie (INSU-CNRS), the Re´gion Lorraine and by the French National Research Agency, contract ANRBS56-008, project Shocks. G.L. is indebted to the INSU-CNRS since part of his work was conducted at the HIGP (Honolulu, USA) as a CNRS delegate. This is CRPG-CNRS contribution #2186.

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